1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system for an internal combustion engine equipped with an evaporative emission control system for purging (discharging) evaporative fuel generated from a fuel tank into an intake system of the engine to thereby control the emission of evaporative fuel into the air.
2. Prior Art
Conventionally, a control system for an internal combustion engine has been proposed, e.g. by U.S. Pat. No. 4,537,172, filed Oct. 21, 1983 (hereinafter referred to as "the first prior art system"), which comprises a fuel tank, a canister for adsorbing evaporative fuel generated from the fuel tank to temporarily store it therein, a purging passage connecting between the canister and an intake system of the engine, a flowmeter arranged in the purging passage for measuring a flow rate of a purged gas (mixture of evaporative fuel and air), and a purge control valve arranged in the purging passage at a location downstream of the flowmeter for controlling the flow rate of purged gas.
According to the first prior art system, evaporative fuel generated from the fuel tank is temporarily stored in the canister, and the stored evaporative fuel is purged into the intake system of the engine as a fuel component for combustion to be burned in a combustion chamber of the engine together with fuel injected by a fuel injection valve. Further, according to this system, the flow rate of purged gas is controlled by means of the purge control valve arranged in the purging passage so as to control the air-fuel ratio of a mixture supplied to the combustion chamber to a desired value, to thereby prevent the emission of noxious components from the engine into the atmosphere.
Further, a control system for an internal combustion engine (hereinafter referred to as "the second prior art system") has already been proposed e.g. by Japanese Provisional Patent Publication (Kokai) No. 2-245441, in which a required amount of fuel for injection is calculated in response to an output from an air-fuel ratio sensor, and then the required amount of fuel is corrected by decreasing it by an amount of evaporative fuel purged per one rotation of the engine, to control the fuel injection amount based on results of this decremental correction.
Since the second prior art system corrects the required amount of fuel for injection by subtracting the amount of purged fuel therefrom, it is possible to prevent, to some extent, the air-fuel ratio of a mixture supplied to the combustion chamber from transiently deviating from a desired value even if the rotational speed of the engine is drastically changed.
Further, a method of controlling the fuel injection amount (hereinafter referred to as "the third prior art system") has already been proposed e.g. by Japanese Patent Publication (Kokoku) No. 3-59255, which comprises the steps of estimating an amount of fuel adhering to an inner wall surface of an intake pipe and an amount of fuel carried off from the adhering fuel through evaporation to be drawn into a combustion chamber, and determining a required fuel injection amount with these estimated amounts taken into account.
According to the third prior art system, the amount of fuel is corrected so as to compensate for the influence by fuel adhering to the inner wall surface of the intake pipe based on the above-mentioned estimated amounts, and the corrected amount of fuel is injected, so that a correct amount of fuel can be constantly supplied to the combustion chamber, which makes it possible to prevent, to some extent, the air-fuel ratio of a mixture supplied to the combustion chamber from being largely deviated from a desired value.
In the first prior art system, however, the flow rate of evaporative fuel contained in the purged gas varies as a function of the amount of fuel stored in the canister. It is difficult to estimate the amount of evaporative fuel stored in the canister, and hence it is impossible to calculate an accurate amount of purged evaporative fuel, so that even if the flow rate of purged gas is controlled by means of the purge control valve, it is impossible to accurately control the flow rate of purged evaporative fuel. Therefore, when the engine is in a transient state, e.g. when the flow rate of purged gas very largely increases or decreases, or when the engine operating condition drastically changes, the amount of evaporative fuel drawn into the combustion chamber cannot be accurately controlled so that it may be undesirably largely deviated from a desired value, which causes the air-fuel ratio of a mixture supplied thereto to deviate from a desired value, leading to degraded exhaust emission characteristics of the engine.
On the other hand, in the first prior art system, the fuel (gasoline) injected from the fuel injection valves is in a liquid state, and part of the injected fuel adheres to the inner wall surface of the intake pipe, depending on a degree of atomization thereof. On the other hand, the evaporative fuel is in a gaseous state, and hence it is directly drawn into the combustion chamber without adhering to the inner wall surface of the intake pipe. When evaporative fuel, which is directly supplied to the combustion chamber, is purged into the intake system in a large amount, the ratio of the injected fuel to the evaporative fuel actually supplied into the combustion chamber deviates from a desired value. That is, since the evaporative fuel, which is gaseous, is more flammable than the injected fuel, which is liquid, the degree of contribution of the evaporative fuel to combustion and the degree of contribution of the injected fuel to combustion largely deviate from respective desired values.
However, in the first prior art system, the air-fuel ratio feedback control is performed by controlling the total amount of injected fuel and evaporative fuel with the two types of fuel regarded as an indivisible component for combustion. Therefore, under such air-fuel ratio control with the two types of fuel regarded as the indivisible component, it is impossible to control the air-fuel ratio of a mixture to a desired value when evaporative fuel is drawn into the combustion chamber in a large amount.
Further, in the first prior art system, the air-fuel ratio feedback control is performed with the amount of evaporative fuel supplied to the combustion chamber taken into consideration. However, if a large amount of evaporative fuel is purged into the intake system, it is difficult to control the air-fuel ratio of a mixture to a desired value with high accuracy, which causes the emission of a large amount of noxious components such as HC into the atmosphere, resulting in degraded exhaust emission characteristics of the engine.
Further, when the engine has not been warmed up and a catalytic converter arranged in the exhaust system of the engine has not been activated, as is the case with the engine immediately after the start thereof, the engine is generally unable to perform good combustion. Therefore, there has been a strong demand for an engine exhibiting improved exhaust emission characteristics even when the engine is started at a low temperature.
In the second prior art system, there is a delay time period before the air-fuel ratio sensor becomes activated through to generate a proper output signal. Therefore, during the delay time period, it is impossible to calculate a required amount of fuel for injection. In short, it is impossible for the second prior art system to calculate an accurate amount of fuel actually required for injection all the time, particularly when the engine is started. Further, the second prior art system system does not contemplate a time lag caused by transport of fuel to the combustion chamber. Therefore, it is impossible to supply just an actually required amount of fuel to the combustion chamber, making it difficult to constantly control the air-fuel ratio to a desired value.
It is known that gasoline, which is injected fuel, and butane, which is a major component of evaporative fuel, have different stoichiometric air-fuel ratios (the stoichiometric air-fuel ratio of gasoline is approximately 14.6, while that of butane is approximately 15.5), so that the optimum ignition timing depends on the ratio (evaporative fuel ratio) of an amount of evaporative fuel to the total amount of fuel supplied to the combustion chamber.
FIG. 1 shows an example of the relationship between the evaporative fuel ratio (butane/(butane+gasoline)) and the optimum ignition timing .theta.IG, provided that evaporative fuel is 100% butane, in which the solid line indicates the optimum ignition timing .theta.IG assumed when the engine coolant temperature is 87.degree. C. and the dot and chain line that assumed when the engine coolant temperature is 33.degree. C.
As is clear from FIG. 1, as the evaporative fuel ratio increases, the optimum ignition timing .theta.IG shifts in a retarding direction, with the shift degree also depending on the engine coolant temperature.
However, in the first and second prior art systems, the ignition timing .theta.IG control is performed without taking the evaporative fuel ratio into consideration at all. Therefore, if the amount of evaporative fuel purged from the canister increases, the ignition timing deviates from the optimum ignition timing .theta.IG in an advancing direction, which causes degraded exhaust emission characteristics and degraded driveability of the engine.
In the third prior art system, although the amount of fuel injected by the fuel injection valves is corrected so as to compensate for the influence by the adhering fuel, dynamic characteristics of the evaporative fuel purged from the canister, such as the time lag caused by transport of fuel to the combustion chamber, are not taken into consideration, making it impossible to constantly and accurately control the air-fuel ratio of a mixture to a desired value.